Energetic limitation of avian parental effort: Field experiments in the kestrel (Falco tinnunculus)

We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h · d^?1 in flight independent of brood size, and expended an average total daily energy of 382 kJ · d^?1. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g · d^?1), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g · d^?1 · chick^?1). Our food deprivation experiments revealed that male kestrels strongly respond to food shortage in the nest. In the older nestling phase males on average increased their daily rate of food delivery to the nest as a response to experimental food deprivation by almost three times to 646.4 g · d^?1, by increasing their flight activity level from 4.46 to 8.41 h · d^?1. This increased energy expenditure was sustained, for as long as eleven days, by increasing the metabolizable energy intake up to what is presumed to be the maximum rate. Even under considerable experimental food stress (chicks not being satiated due to continuous removal of delivered food by the observers) about half of the available daylight time remained unused for foraging. We conclude 1) that the mean daily energy expenditure of males during nestling care — to which clutch size is apparently initially adjusted — is well below the maximum they are able to sustain and 2) that the energy expenditure they can sustain under extremely high nestling demand is not set by the available time for foraging or the available energy in the environment. Thus the birds normally operate well below their presumed maximum, and only during food shortage, e.g., as caused by our experiments, do they increase activity up to this maximum. Therefore we conclude that the kestrels have costs other than energy expenditure, such as parental survival, that are involved in the increased “cost” of parental effort. We discuss possible generalisations about existing energetic limitations during parental care in altricial birds. From published estimates of daily energy expenditure during parental care (DEE_par) in 30 different bird species we derived the equation: DEE_par = 14.26 kg^0.65 Watt. This relationship differs significantly in slope (T = 2.49; p > 0.02) from the allometric equation for the maximum rate of energy assimilation (DME_max) as provided by Kirkwood (1983): DME_max = 19.82 kg^0.72 Watt. In smaller species (ca. 25 g) DEE_par about equals DME_max, while in the larger species (ca. 10 kg) DEE_par represents only about 60% of the predicted DME_max. This suggests that limitations in parental effort are more frequently set by the maximum sustainable energy intake in the smaller species than in larger species. Our allometric equations for DEE_par suggests that the relation between BMR, estimated using the equations of Aschoff and Pohl (1970), and the observed parental energy expenditure, is such that on average bird parents work at a daily level somewhere between 3 and 4 times BMR.     

Individual variation in field metabolic rate of kittiwakes (Rissa tridactyla) during the chick-rearing period

Field metabolic rate (FMR), using the doubly labelled water (DLW) method, was measured in free-ranging adult kittiwakes (Rissa tridactyla) early and late in the chick-rearing period at Svalbard, Norway. Individual variation in FMR was analysed by comparing FMR with body mass, sex, nest attendance, chick age, brood size, and basal metabolic rate (BMR). Mean FMR of kittiwakes during the chick-rearing period was 27.0+/-0.9 (SE) W kg(-1), while the individual variation (calculated as coefficient of variation [CV]) in FMR was 24%. Sex, time spent away from the nest, age of the chicks, and brood size contributed significantly to FMR and explained 65% of the variation in FMR. The FMR increased by 32% from early until late in the chick-rearing period. This occurred simultaneously with an increase in the time spent away from the nest. In 15 of 20 pairs, one of the mates had 15% or higher (mean of the 15 pairs, 22%+/-8%) FMR (W kg(-1)) than their partner, even though the mates spent equal amounts of time away from the nest. This indicates an intrapair conflict in FMR. The variation in total FMR of pairs was 40% less than the individual variation, and total FMR of pairs increased with age of the chicks. This indicates that the mates adjust their energy expenditure within a relatively constant FMR determined by the energy needs of the chicks. Individual variation in FMR could not be explained by variation in body mass or BMR. BMR measured late in the chick-rearing period was 26% lower than previous measurements of BMR from the prebreeding and incubation periods. The increase in FMR and simultaneous decrease in BMR caused a 40% increase in metabolic intensity (FMR/BMR) of kittiwakes during the chick-rearing period. It is suggested that the metabolic intensity is not a proper measure of the metabolic load in seabirds.     

Exploring individual quality: basal metabolic rate and reproductive performance in storm-petrels

Despite evidence that some individuals achieve both superiorreproductive performance and high survivorship, the factorsunderlying variation in individual quality are not well understood.The compensation and increased-intake hypotheses predict thatbasal metabolic rate (BMR) influences reproductive performance;if so, variation in BMR may be related to differences in individualquality. We evaluated whether BMR measured during the incubationperiod provides a proximate explanation for variation in individualquality by measuring the BMRs and reproductive performance ofLeach's storm-petrels (Oceanodroma leucorhoa) breeding on KentIsland, New Brunswick, Canada, during 2000 and 2001. We statisticallycontrolled for internal (body mass, breeding age, sex) and external(year, date, time of day) effects on BMR. We found that maleswith relatively low BMRs hatched their eggs earlier in the seasonand that their chicks' wing growth rates were faster comparedto males with relatively high BMRs. Conversely, BMR was notrelated to egg volume, hatching date, or chick growth rate forfemales or to lifetime (23 years) hatching success for eithersex. Thus, for males but not for females, our results supportthe compensation hypothesis. This hypothesis predicts that animalswith low BMRs will achieve better reproductive performance thananimals with high BMRs because they have lower self-maintenancecosts and therefore can apportion more energy to reproduction.These results provide evidence that intraspecific variationin reproductive performance is related to BMR and suggest thatBMR may influence individual quality in males.     

Basal metabolic rate is positively correlated with parental investment in laboratory mice

The assimilation capacity (AC) hypothesis for the evolution of endothermy predicts that the maternal basal metabolic rate (BMR) should be positively correlated with the capacity for parental investment. In this study, we provide a unique test of the AC model based on mice from a long-term selection experiment designed to produce divergent levels of BMR. By constructing experimental families with cross-fostered litters, we were able to control for the effect of the mother as well as the type of pup based on the selected lines. We found that mothers with genetically determined high levels of BMR were characterized by higher parental investment capacity, measured as the offspring growth rate. We also found higher food consumption and heavier visceral organs in the females with high BMR. These findings suggested that the high-BMR females have higher energy acquisition abilities. When the effect of the line type of a foster mother was controlled, the pup line type significantly affected the growth rate only in the first week of life, with young from the high-BMR line type growing more rapidly. Our results support the predictions of the AC model.     

树麻雀代谢率和器官重量在季节驯化中表型的可塑性变化

动物能量代谢的生理生态特征与物种的分布和丰富度密切相关,基础代谢率(BMR)是内温动物能量预算的重要组成部分。北温带的小型鸟类,通过增加产热来适应低温环境。增加BMR的基础之一是中心器官(代谢机器)发生明显的变化。本研究中我们测定了树麻雀(Passermontanus)的BMR、体重和各器官的重量,分析了麻雀各器官的季节性变化及与BMR的关系。方差分析表明:麻雀的BMR存在明显的季节性变化,在冬季和秋季较高。麻雀内部器官的变化同样有明显的季节性,冬季和秋季麻雀的肝脏、心脏、肌胃、小肠、直肠和整体消化道的重量,都有明显的增加。相关分析表明:麻雀的BMR与肝脏、心脏和消化道等内部器官存在明显的相关性。我们的结果验证了“中心限制假说”,即麻雀体内存在着与BMR相关的“代谢机器”,中心器官是提高麻雀BMR的基础之一。     

Moult and basal metabolic costs in males of two subspecies of stonechats: the European Saxicola torquata rubicula and the East African S. t. axillaris

The circannual patterns in resting metabolic rate (RMR) of males of two subspecies of stonechats, the European Saxicola torquata rubicula and the East African S. t. axillaris, are compared. As the birds from the two subspecies were raised and kept under comparable laboratory conditions, differences in metabolic rate between the two subspecies had to be genetically determined. RMR peaked during moult in both subspecies. During the rest of the year RMR was fairly constant in both subspecies and assumed to reflect basal metabolic rate (BMR). African stonechats had a 22% lower mass specific BMR than European stonechats, which is thought to reflect a genetical physiological adaptation to the differences in environmental circumstances they experience in the field. A low BMR makes an animal more susceptible to cold. Hence, the relatively high plumage mass in the African compared to the European stonechat may be functionally linked to its relatively low BMR. Moult costs, calculated from the plumage masses and the differences in RMR inside and outside the moult period, tended to be higher in the European compared to the African stonechats. These data and an interspecific comparison of moult costs over various species of birds support the earlier notion by Lindström et al. (1993) that moult costs are more closely linked with BMR than with body mass or rate of moult. The relation between moult costs and BMR and the fact that the efficiency of moult is extremely low (3.8 and 6.4% for European and African stonechats, respectively) suggest that the maintenance of specific tissues necessary for moult is a large cost factor. Alternatively, impaired insulation during moult may necessitate an increased metabolic capacity which may be associated with an increased RMR.     

Quantitative genetics and fitness effects of basal metabolism

The physiological requirements of reproduction are predicted to generate a link between energy, physiology and life history traits. Simultaneously, low maintenance costs, measured by energy consumption, are expected to be advantageous. Here we investigated fitness relatedness of traits by estimating genetic correlations between, and inbreeding depression for, body mass, basal metabolic rate (BMR) and other life history characters in a wild rodent, Myodes glareolus. The narrow-sense heritability of absolute and mass corrected BMRs were high for females (h² = 0.48 and 0.42) but low and non-significant for males (0.32 and 0.09). A significant positive genetic correlation between BMR and litter size suggests that traits connected to female fecundity might favour higher metabolism (i.e. support increased intake hypothesis). However, the estimates of inbreeding depression indicate that, while higher values of body mass and female litter size could be positively associated with overall fitness, the association between BMR and overall fitness in bank voles would be negative (i.e. support compensation hypothesis). This result suggests that the advantages of larger litters and larger body mass might be evolutionary constrained by high costs of maintenance of those traits, as reflected by the level of basal metabolism.     

The energy economy of the arctic-breeding Kittiwake (Rissa tridactyla): a review

The present paper reviews recent studies on changes in body mass, body composition and rates of energy expenditure during the breeding season in the black-legged Kittiwake (Rissa tridactyla) on Svalbard (79 degrees N). The main characteristic of the energy budget is a pronounced decrease in body mass as well as basal metabolic rate (BMR) after the eggs have hatched. While most internal organs lose mass in direct proportion to the general decrease in body mass, the liver and kidney masses decrease to a disproportionately greater extent. Since both the liver and the kidney have high intrinsic metabolic rates, these results support an earlier notion that the reduction in body mass is an adaptation to reduce maintenance costs. Alternatively, the reduced BMR is due to a decrease in energy uptake from the gastrointestinal tract, thereby ensuring that undigested food is ready to be regurgitated to the chicks. At the end of the chick-rearing period, the field metabolic rate (FMR) reaches its highest level, probably due to an increased workload associated with chick feeding. This occurs at a time of low body mass and BMR. A pronounced increase in the metabolic scope (FMR/BMR) during the latter part of the chick-rearing period demonstrates that BMR and FMR may change independently of each other and that the ratio FMR/BMR may not be a good measure of energy stress.     

Basal metabolic rate and risk‐taking behaviour in birds

Basal metabolic rate (BMR) constitutes the minimal metabolic rate in the zone of thermo‐neutrality, where heat production is not elevated for temperature regulation. BMR thus constitutes the minimum metabolic rate that is required for maintenance. Interspecific variation in BMR in birds is correlated with food habits, climate, habitat, flight activity, torpor, altitude, and migration, although the selective forces involved in the evolution of these presumed adaptations are not always obvious. I suggest that BMR constitutes the minimum level required for maintenance, and that variation in this minimum level reflects the fitness costs and benefits in terms of ability to respond to selective agents like predators, implying that an elevated level of BMR is a cost of wariness towards predators. This hypothesis predicts a positive relationship between BMR and measures of risk taking such as flight initiation distance (FID) of individuals approached by a potential predator. Consistent with this suggestion, I show in a comparative analysis of 76 bird species that species with higher BMR for their body mass have longer FID when approached by a potential predator. This effect was independent of potentially confounding variables and similarity among species due to common phylogenetic descent. These results imply that BMR is positively related to risk‐taking behaviour, and that predation constitutes a neglected factor in the evolution of BMR.     

小鼠基础代谢率与繁殖输出和繁殖期末器官的关系

以封闭式流体压力呼吸计测定KM小鼠(Mus musculus)的基础代谢率(BMR);采用残差分析和Pearson相关分析检验BMR与繁殖输出、内脏器官的相关性。哺乳末期BMR显著高于繁殖前,繁殖前BMR与繁殖输出不相关,但哺乳末期BMR与体重、摄食量、胎仔数和胎仔重、内脏器官和消化道显著正相关;与消化道器官的相关性高于其他内脏器官。研究结果支持"哺乳期较高的BMR有利于消化系统增强消化和吸收能力,以增加能量摄入用于繁殖输出"的假设。     

Dominant black-capped chickadees pay no maintenance energy costs for their wintering status and are not better at enduring cold than subordinate individuals

Winter requires physiological adjustments in northern resident passerines. Cold acclimatization is generally associated with an increase in physiological maintenance costs, measured as basal metabolic rate (BMR), and cold endurance, reflected by summit metabolic rate (M _sum). However, several northern species also form social groups in winter and a bird’s hierarchical position may influence the size of its metabolically active organs as well as its BMR. Winter metabolic performance in these species may therefore reflect a complex set of adjustments to both seasonal climatic variations and social environment. We studied the effect of social status on parameters of cold acclimatization (body mass, size of fat reserves and pectoral muscles, BMR and M _sum) in free-living black-capped chickadees (Poecile atricapillus). Birds that were structurally large and heavy for their body size, mostly dominant individuals, carried more fat reserves and had larger pectoral muscles. However, social status had little effect on metabolic performance in the cold. Indeed, M _sum was independent of social rank while mass-corrected BMR was slightly lower in dominant individuals, likely due to a statistical dilution effect caused by large metabolically inactive fat reserves. BMR and M _sum, whether considered in terms of whole-animal values, corrected for body mass or body size were nevertheless correlated, suggesting a functional link between these metabolic components. Our results therefore indicate that the energy cost of social dominance is not a generalized phenomenon in small wintering birds.     

Basal metabolic rate in carnivores is associated with diet after controlling for phylogeny

Studies of basal metabolic rate (BMR), the minimum metabolic rate of postabsorptive, inactive endotherms while in their rest phase and thermal neutral zone, have contributed significantly to our understanding of animal energetics. Besides body mass, the main determinant of BMR, researchers have invoked diet and phylogenetic history as important factors that influence BMR, although their relative importance has been controversial. For 58 species within the Carnivora, we tested the hypothesis that BMR is correlated with home range size, a proxy for level of activity, and diet, using conventional least squares regression (CLSR) and regression based on phylogenetic independent contrasts (PIC). Results showed that BMR of Carnivora was positively correlated with home range size after controlling for body mass, regardless of the statistical method employed. We also found that diet and mass-adjusted home range size were correlated. When we simultaneously tested the effect of diet and mass-adjusted home range on mass-adjusted BMR, home range size was insignificant because of its colinearity with diet. Then we eliminated home range size from our model, and diet proved to be significant with both CLSR and PIC. We concluded that species that eat meat have larger home ranges and higher BMR than species that eat vegetable matter. To advance our understanding of the potential mechanisms that might explain our results, we propose the "muscle performance hypothesis," which suggests that selection for different muscle fiber types can account for the differences in BMR observed between meat eaters and vegetarian species within the Carnivora.     

Costs and benefits of parental care in eastern kingbirds

We manipulated brood sizes of eastern kingbirds (Tyrannus tyrannus)to measure the costs and benefits of parental care and to testwhether kingbirds showed evidence of individual optimizationof reproductive effort. We found that the number of feedingtrips (trips/h) increased and that per capita feeding rates(trips/nestling/h) declined as brood size increased. The declinein per capita feeding rates was mostly due to high feeding rateto broods of one: parents made roughly equal number of tripsto feed each nestling in broods of two to five. Nonetheless,nestling mass declined with brood size, probably because largebroods were fed more small prey. Nestling condition (mass adjustedfor structural size) differed only between broods of one andfive. After controlling for effects of brood size, feeding rateshad no supplementary influence on either nestling size or condition,but productivity and feeding rate were positively and significantlyrelated. Adult male condition did not vary with brood size,manipulated brood size, or total feeding rate, but declinedas the pair's per capita feeding rates increased. In addition,males that returned to breed were in better condition beforeleaving for migration than those that failed to return. Femalecondition tended to decline, and the probability of returningto breed dropped when broods were enlarged. However, femalecondition was independent of the probability of returning. Ourresults show that high feeding rates were costly, but that theycarried benefits (greater productivity). Some evidence for individualoptimization of reproductive effort existed: variability innestling and adult female condition were better explained bychanges in brood size than by the actual number of young inthe nest. However, most evidence supported the alternative thatincreased brood size was equally costly for all birds     

Parental fish change their cannibalistic behaviour in response to the cost-to-benefit ratio of parental care

Partial filial cannibalism, the act of cannibalizing some offspring, has been explained as a response to the high energetic cost of care. I tested this hypothesis by manipulating the cost-to-benefit ratio of care in the scissortail sergeant, Abudefduf sexfasciatus, a tropical damselfish with male care. Background egg mortality was lower than the incidence of cannibalism, confirming that males did not just dispose of dead eggs. Investment in the current brood affected future investment, because males forced to skip a brood cycle put more effort into courtship during the following cycle and obtained larger broods than did unmanipulated males. Any factor influencing the cost-to-benefit ratio of parental care should also affect the incidence of cannibalism. I reduced the cost of care by supplementary feeding and reduced the benefit of care by simulating a decrease in paternity certainty through simulated intrusions by non-nesting males. Supplementary feeding significantly reduced partial filial cannibalism by parental males, a result compatible with the hypothesis that eggs are consumed to cover the energetic costs of parental care. Cannibalism decreased regardless of whether males were fed with conspecific eggs or crabmeat. Cannibalism was only reduced but not fully eliminated by supplementary feeding, and residual levels of cannibalism after feeding were similar to the background rate of egg mortality. Simulated intrusions by non-nesting males led to an increase in filial cannibalism and a decrease in parental effort.     

A test of the thermal-stress and the cost-of-burrowing hypotheses among populations of the subterranean rodent Spalacopus cyanus

Subterranean mammals show lower than-allometrically expected-basal metabolic rates (BMR), and several competing hypotheses were suggested to explain how physical microenvironmental conditions and underground life affect subterranean mammalian energetics. Two of these are the thermal-stress and the cost-of-burrowing hypotheses. The thermal-stress hypothesis posits that a lower mass-independent BMR reduces overheating in burrows where convective and evaporative heat loss is low, whereas the cost-of-burrowing hypothesis states that a lower mass-independent BMR may compensate for the extremely high energy expenditure of digging during foraging activity. In this paper we tested both hypotheses at an intraspecific level. We compared seven populations of the subterranean rodent Spalacopus cyanus or cururo from different geographic localities with contrasting habitat conditions. We measured BMR and digging metabolic rate (DMR) through open flow respirometry. Our results support neither the thermal-stress nor the cost-of-burrowing hypothesis. Cururos from habitats with contrasting climatic and soil conditions exhibited similar BMR and DMR when measured under similar semi-natural conditions. It is possible that S. cyanus originated in Andean locations where it adapted to relatively hard soils. Later, when populations moved into coastal areas characterized by softer soils, they may have retained the original adaptation without further phenotypic changes.     

Metabolic adjustments in breeding female kittiwakes (<Emphasis Type="Italic">Rissa tridactyla</Emphasis>) include changes in kidney metabolic intensity

Black-legged kittiwakes (BLKIs) reduce self-maintenance cost through reductions in mass-specific basal metabolic rate (BMR), body mass and the size of visceral organs during the chick-rearing period. In the present study, we measured kidney in vitro oxygen consumption and plasma 3,3',5-triiodo-L: -thyronine (T3) levels of incubating and chick-rearing female BLKIs, to test whether the decrease in BMR is caused mainly by decreased metabolic intensity or simply by reductions in the size of organs with high metabolic intensity. Body mass and body condition were lower in chick-rearing birds compared with the incubating birds. In contrast to the previous findings, however, the kidney mass did not differ between the two breeding stages. Plasma T3 levels decreased substantially during the breeding season, indicating a reduction in BMR. Over the same period, kidney mass-specific oxygen consumption decreased (by 17.2%) from the incubating to the chick-rearing stage. Thus, the reduction in BMR found in breeding BLKIs seems partly explained by adjustments in metabolic intensity of visceral organs. Lowered metabolic intensity of visceral organs would permit increased allocation of energy to offspring at the expense of their own self-maintenance.     

The functional significance of individual variation in basal metabolic rate

Basal metabolic rate (BMR) was established as a common reference point allowing comparable measures across different individuals and species. BMR is often regarded as a minimal rate of metabolism compatible with basic processes necessary to sustain life. One confusing aspect, however, is that BMR is highly variable, both within and between species. A potential explanation for this variability is that while individuals with high BMRs may suffer the disadvantage of having to feed for longer to cover the extra energy demands, this may be offset by advantages that accrue because of the high metabolic rate. One suggested advantage is that high levels of BMR are a consequence of maintaining a morphology that permits high rates of the maximal sustained rate of metabolism (SusMR)--the rate of metabolism that can be sustained for days or weeks. We have been studying the energetics of MF1 laboratory mice during peak lactation to investigate this idea. In this article, we review some of our work in connection with three particular predictions that derive from the hypothesised links among morphology, basal metabolism, and sustained metabolic rate. By comparing groups of individuals, for example, lactating and nonlactating individuals, the patterns that emerge are broadly consistent with the hypothesis that BMR and SusMR are linked by morphology. Lactating mice have bigger organs connected with energy acquisition and utilisation, greater resting metabolic rates in the thermoneutral zone, called RMRt (approximately equivalent to BMR), and high sustainable rates of maximal energy intake. However, when attempts are made to establish these relationships across individuals within lactating mice, the associations that are anticipated are either absent or very weak and depend on shared variation due to body mass. At this level there is very little support for the suggestion that variation in RMRt (and thus BMR) is sustained by associations with SusMR.     

Cost of reproduction, T-lymphocyte mediated immunocompetence and health status in female and nestling barn swallows Hirundo rustica

We investigate the trade-off between reproductive effort, health status and T-lymphocyte acquired immunity in female and nestling barn swallows Hirundo rustica using a brood size manipulation experiment. Maternal and total feeding effort increased with experimental brood size. Parents did not fully compensate for the increased food demand of the enlarged broods and as a consequence the per capita feeding rate of nestlings decreased with increasing experimental brood size. Body mass and a measure of T-cell mediated immunity in 12 days old nestlings also decreased with increasing experimental brood size. Different leucocyte concentrations and the heterophile/lymphocyte ratio – an index of stress – of nestlings did not change in relation to experimental brood size, suggesting that within brood competition did not affect stress to nestlings. The brood size manipulation had a significant effect on maternal T-cell mediated immunity, measured by the phytohemagglutinin skin test, but not on maternal body mass, haematocrit or differential or total white blood cell counts. Our results seem to support the prediction that under mild work stress females respond first by reducing the energetically expensive acquired immunity. Different leucocyte types and the heterophile/lymphocyte ratio appear less sensitive to parental workload.     

Metabolic costs of brain size evolution

In the ongoing discussion about brain evolution in vertebrates, the main interest has shifted from theories focusing on energy balance to theories proposing social or ecological benefits of enhanced intellect. With the availability of a wealth of new data on basal metabolic rate (BMR) and brain size and with the aid of reliable techniques of comparative analysis, we are able to show that in fact energetics is an issue in the maintenance of a relatively large brain, and that brain size is positively correlated with the BMR in mammals, controlling for body size effects. We conclude that attempts to explain brain size variation in different taxa must consider the ability to sustain the energy costs alongside cognitive benefits.     

Energetics of growth in House martins (Delichon urbica)

Energy requirements for growth and maintenance of nestling House martins was studied in relation to brood size and age. Parallel studies of feeding rates and faecal output were made. The effect of mutual insulation between members of the brood on metabolism was of significance for reducing maintenance energy requirements. Of great value for predicting the peak energy demand of the brood was the number of young and their individual requirements for growth and maintenance at different ages. Power equations are given for predicting brood assimilation, faecal output and feeding frequency in relation to brood size. The impact of food scarcity on growth patterns was assessed. The large lipid stores of nestlings served as an energy reserve during adverse conditions both before and after fledging. Asynchronous hatching in large broods is interpreted as a mechanism which serves to minimize the peak in energy demand which occurs about 12 days after the brood hatches.